This invention generally relates to methods for making electrical connections. In particular, the invention relates to methods for electrically connecting one electrical conductor to another when one or both of the electrical conductors have a gold surface.
Flexible printed circuits (“flex circuits”) and numerous other metallized substrates often have gold as the final metal layer exposed to the ambient atmosphere. The gold surface is very stable and therefore is typically not changed during storing. However, the same physical property that provides for excellent environmental stability also makes bonding to the gold surface very difficult. It is well known in the art that the level of adhesion which forms between an epoxy and the surface of a gold-plated flex circuit is lower than that which forms between an epoxy and many other metals. However, because of its excellent environmental stability, gold continues to be a preferred final metal layer for many applications.
Some ultrasound transducers are manufactured by bonding a flex circuit to the surface of the piezoelectric ceramic prior to dicing of the individual piezoelectric elements. In many cases gold is selected as the final metal layer for both the surface of the ceramic and the surface of the flex circuit. A common process used to connect the flex circuit to the piezoelectric ceramic is lamination with a thin epoxy bond. Low bond strength across the ceramic-to-flex circuit bond can lead to delamination of the flex circuit from the piezoelectric ceramic during the dicing process or other processing step, resulting in a nonfunctioning piezoelectric element. Therefore it is desirable to substantially increase the strength of this epoxy bond to the gold surface.
Several commercial primers are available for treating the gold surface to promote bonding. Many of these primers are sulfur-containing alkoxysilanes. These primers increase the bond strength of an epoxy bonded to the gold surface relative to either no primer or a non-sulfur-containing alkoxysilane primer when used in the recommended surface treatment method
There is a need for means for further increasing the epoxy-to-gold bond strength of gold-to-other metal and gold-to-gold electrical connections.